1. Field of the Invention
The present invention relates to an apparatus and method for measuring the rheological properties, e.g., shear stress, shear strength, die swell ratio and the like, as well as rheological changes which occur during thermoplastic foam formation and extrusion of such foam through an extrusion orifice.
2. Description of the Prior Art
In the past, activity has been reported in the literature concerned with the measurement of the rheological properties of foamed plastics (C. G. Benning, Plastics Foams, Vol. I, Wiley-InterScience, New York 1969) and molten polymers containing filler materials such as extenders (C. D. Han, Journal of Applied Polymer Science, 18, 821-829 -- 1974). In such work, the rheological characterization of polymeric materials or composites is an important step in studying the various problems encountered during the processing and manufacturing of such materials during commercial operations. Due to diffusion phenomena in a highly viscous polymer melt, it is difficult to obtain flow data on a two-component mixture, one of which is a volatile liquid, e.g. molten polymers in admixture with volatile blowing agents such as isopentane, by using existing commercially available melt rheometers. Generally, in the foamed plastics areas, most of the prior art dealing with the rheological changes which occur during thermoplastic foam formation have been restricted to trial and error experimental approaches because of the lack of proper instrumentation.
Up to the present time, the most common prior art methods to evaluate molten thermoplastics containing a blowing agent have included:
(a) heating a mixture of a solid blowing agent and an inert liquid, and measuring the gaseous products which are liberated; PA1 (b) the utilization of differential thermal analysis which measures the heat of the reactive exotherm when solid blowing agents are employed; PA1 (c) thermal gravimetric analysis for measuring weight loss as a function of temperature (utilizing solid or liquid blowing agents); and PA1 (d) the employment of mixers such as a miniature Banbury, to measure the viscous properties of a polymeric melt as calculated from torque measurements.
The preceding methods (a), (b) and (c) are deficient in that the blowing agent decomposition behavior is determined under conditions which bear no relation to actual extrusion processes. With respect to method (d), the measurement of elastic turbulance of this type can not be directly related with processability.
Accordingly, for accurate determination of rheological properties, a standard rheometer device such as the device described in ASTM Test Designation -- D 1238 -- 57T, is still to be preferred for the development of research data and to assist in die design. However, such standard or typical capillary rheometer as employed in the prior art have no provision for making flow measurements on melts containing volatile liquids, i.e. blowing agents. Such devices are intended to employ starting materials which are in the form of powder, granules, strips of film or molded slugs. Such materials may be easily heated to a molten condition utilizing a heater jacket surrounding the standard rheometer device and flow measurement may be subsequently determined by extrusion of molten materials through a capillary orifice. However, it will be appreciated that a molten thermoplastic with a volatile blowing agent or an additive admixed therein constitutes a feed material which could not be readily employed with such standard devices.
The elasticity and flow characteristics of polymer melt is a major concern for the processing of plastics. These elastic effects manifest themselves in a number of ways during polymer processing such as melt fracture, sharkskin, frozen-in orientation and the like are a few examples. It has been found that these elastic effects are related to the die swell observed in the melt flow measurements. Therefor die swell measurement together with the shear stress and the shear rate measurements will provide a considerable amount of background information concerning the processability of the blowing agent in admixture with a polymer melt filled polymer and polymer composites.